CILIA AND FLAG ELL A. 27 



Their size, in proportion to that of the entire cell, also 

 varies; usually the nucleus comprises but a small part of the 

 corpuscle ; but in some cases, as in the mononuclear leukocyte, 

 the nucleus occupies the largest part of the cell. 



Most animal and vegetable cells are nucleated. Nuclei 

 have not been demonstrated in some of the unicellular organ- 

 isms, however, though they are probably present. Among 

 mammalian cells one important class, the red blood- corpuscles, 

 is not nucleated. The nucleus seems such an essential feat- 

 ure in the life and reproduction of cells that cells, like the red 

 blood-corpuscles, that are not nucleated are probably lacking 

 in the full attributes of vitality. 



The precise function of the nucleus is not clear ; but it is 

 evidently of high importance, especially in connection with 

 reproduction and heredity. It is probably the most vital 

 part of the cell, the body largely serving nutritive and me- 

 chanical purposes. 



The centrosome, or polar corpuscle, is a minute round re- 

 fractive body or point, situated in the interior of the cell. 

 The protoplasmic granules around it are arranged in delicate 

 radiating lines, forming the attraction-sphere. In resting or 

 non-dividing cells it may be within the nucleus, or just out- 

 side ; but in this stage it is not conspicuous and is usually in- 

 visible. During the process of karyokinetic cell-division it 

 appears prominently in the protoplasm of the cell-body, and 

 it plays an important part in this process, apparently exercis- 

 ing some sort of polar, or directive, or attractive influence in 

 the division. 



The cell-wall, or cytolemma : Many cells are surrounded with 

 a covering or wall, which gives support and protection to the 

 soft protoplasm. This may be a distinct membrane, or a con- 

 densation and hardening of the outermost layer of protoplasm. 

 Some of the lower unicellular organisms secrete around them- 

 selves a silicious or calcareous covering. The cell-walls of 

 plants consist of cellulose, a firm substance belonging to the 

 starch group, to which the hardness of wood is due. Some 

 cells, as the leukocytes, appear to be devoid of a distinct cell- 

 wall. 



Cilia and flagella : Certain kinds of cells have fine, hair-like 

 processes or filaments projecting from their surface, capable 



